The document about kidnapping includes an extra line of text if you've met the King of the Fae:
One of the descriptions sounds like that child you met in the cave, although the sketch doesn't bare any resemblance.
("bare" typo reported)
Could be worthwhile to get this message and then return to the Fairy King to see if you can rescue the child. But the options available in that encounter don't really involve the kid (and the kid seems to disappear if you trap the fae) so I'm not optimistic.
In the beginning, men were mortal.